Andrius Zukauskas

• PhD
• Researcher at KTH Royal Institute of Technology

Microlaser-pumped subnanosecond pulse duration optical parametric generators have great potential to establish themselves as a compact, simple, and cost-effective option for various tasks that do not require high temporal resolution. However, fundamental limitations of co-propagating three-wave interaction, including broad bandwidth, non-transform-...

The first demonstration of a 2.7 µm CO2 gas sensing source exploiting a backward wave optical parametric oscillator (BWOPO). Transmission measurements of the backward wave are demonstrated through air with good agreement with simulations.

Backward wave oscillators represent a class of tunable sources of electromagnetic radiation that do not require a resonant cavity to satisfy the oscillation condition. In the optical regime, the Backward Wave Optical Parametric Oscillator (BWOPO) relies on a a nonlinear interaction to provide the positive feedback required for oscillation, achieved...

We report on a cascade nanosecond CdSe optical parametric oscillator with an output energy of 0.54 mJ at 100 Hz, temperature tuning within 9245–9285 nm, and a narrow (∼6 cm⁻¹) spectral width achieved by a volume Bragg grating acting on the first stage. The CdSe crystal is pumped intracavity by the signal wave from the PPKTP-based first stage, and b...

Spectral narrowing of the first stage signal wave in a cascade optical parametric oscillator (OPO) using a volume Bragg grating produced narrowband ${\sim}{\rm 0.6 \text{-} mJ}$ idler pulses at 9.245 µm from the second stage based on CdSe at a repetition rate of 100 Hz. The CdSe crystal is pumped intracavity by the signal wave from the periodically...

In this work, a novel 2.7 µm source used for CO2 and H2O vapor spectroscopy using the backward propagating wave of a backward wave optical parametric oscillator (BWOPO) is demonstrated for the first time to our knowledge. The unique properties of BWOPOs eliminate the need for additional spectral narrowing or wavelength stabilization, enabling the u...

In this study, we demonstrated the direct use of an inherently narrowband backward wave in the mid-infrared for CO$_2$ and H$_2$O vapor spectroscopy in ambient air. This wave is generated in a backward wave optical parametric oscillator (BWOPO) pumped by a multi longitudinal mode laser at 1030 nm, eliminating the need for additional spectral narrow...

Backward wave oscillators (BWO) represent a class of tunable sources of electromagnetic radiation that do not require a resonant cavity to satisfy the oscillation condition. Electronic BWOs are widely used as high-power sources of microwave radiation. In the optical regime the backward wave optical parametric oscillator (BWOPO) rely on a conceptual...

We investigate a new method of coercive field engineering for periodic poling of RbKTiOPO4 (RKTP). By ion exchanging RKTP in a molten salt containing 7 mol% Ba(NO3)2 and 93 mol% KNO3 we achieve more than an order of magnitude difference in polarization switching time between the exchanged and non-exchanged regions. This method is used to fabricate...

Spectral narrowing of the first stage signal wave in a cascade optical parametric oscillator using a Volume Bragg Grating produced narrowband ⁓0.6-mJ idler pulses near 9.26 μm from the second stage.

We present backward wave optical parametric oscillator waveguides implemented in periodically poled Rb-doped KTP. The waveguides demonstrated low loss (0.16 dB/cm) and exhibited an oscillation threshold 19 times lower than the corresponding bulk device.

For the first time, we show spectral and polarization indistinguishability of photon pairs generated in counter-propagating degenerate spontaneous parametric downconversion in the telecommunications band using first-order quasi-phase-matching in periodically poled Rb-KTiOPO 4 .

We report on a noncritically phase-matched cascade CdSe optical parametric oscillator with an output energy of 0.54 mJ at 100 Hz, temperature tuning within 9274-9314 nm, and narrow spectrum achieved by a Volume Bragg Grating.

We demonstrate the first Mirrorless Optical Parametric Oscillator based on Rb:PPKTP crystal with 427 nm grating period and pumped by subnanosecond pulses from Nd:YAG microlaser achieving nearly transform-limited pulses and diffraction-limited beams of the signal and idler waves.

We demonstrate a continuously tunable mid-infrared source that produces narrowband radiation at 1981 nm and 2145 nm based on a tunable Yb-based hybrid MOPA pump and a backward-wave optical parametric oscillator (BWOPO). The BWOPO employs a PPRKTP crystal with 580 nm domain periodicity. The BWOPO has a record-low oscillation threshold of 19.2 MW/cm2...

Backward-wave three-wave mixing is a difficult χ⁽²⁾ interaction to observe, because it requires ultrashort poling periods to achieve phasematching. Having realized sub-micrometer periods in periodically poled KTiOPO4 (PPKTP), we demonstrate for the first time first-order quasi-phasematched, backward-wave spontaneous parametric downconversion (BW SP...

A stable, narrow-bandwidth (274 MHz) backward wave optical parametric oscillator (BWOPO) generating mJ-level backward signal at 1885nm and forward idler at 2495 nm is presented. The BWOPO was pumped by a single-longitudinal mode, Q-switched Nd:YAG high-energy laser at 1064 nm. We show that multi-transversal mode pumping leads to the spectral broade...

We demonstrate periodic poling of large aperture Rb-doped KTP (RKTP) crystals with a QPM period of 3.43 µm via coercive field engineering. The periodically-poled RKTP has excellent pattern-fidelity, with a second harmonic generation (SHG) conversion efficiency of 1.4%/Wcm at 405 nm. We use ion-exchange (IE) to fabricate grating of high-and-low coer...

We demonstrate first-order quasi-phase-matched backward second-harmonic generation (BSHG) with an efficiency of 18.7%. This represents an increase by two orders of magnitude from earlier experiments employing higher-order quasi-phase-matching. The efficient BSHG is demonstrated in bulk periodically poled Rb:KTiOPO4 with a poling period of 317 nm. U...

We report a self-phase-locked and degenerate backward wave optical parametric oscillator (BWOPO) generating counterpropagating signal and idler in the spectral band of optical communication. The device is based on a periodically poled Rb-doped KTiOPO 4 crystal with a poling period of 433 nm. To the best of our knowledge, that is the shortest quasi-...

Large aperture periodically-poled Rb:KTP crystals designed for optical parametric amplifiers in 2 µm LIDAR systems were radiation hardness tested by exposure of proton beams at 10 MeV and 60 MeV energies. An irradiation dose of 55 Gy was used to commensurate the crystals’ estimated exposure on board a mission in the low-Earth orbit. The irradiation...

A non-resonant optical parametric oscillator based on 5-mm-thick Rb-doped periodically-poled KTiOPO 4 (PPKTP) generates narrow-band (<1-nm) idler pulses of 5.84-mJ energy at ~2382 nm and 100 Hz using a volume Bragg grating for the signal wave.

We experimentally investigate using backward-wave spontaneous parametric downconversion for frequency translation, where spectral characteristics of the pump wave are transferred to the signal wave.

We present a tunable (6.62-11.34 µm), singly-resonant, cascade optical parametric oscillator with intracavity pumping of BaGa 2 GeSe 6 in the second stage and spectral narrowing realized by a Volume Bragg Grating, providing millijoule output at 100 Hz.

We experimentally explored the design parameter space for periodically poled ${\rm Rb}:{{\rm KTiOPO}_4}$ (PPRKTP) structures in type-II optical parametric oscillators (OPOs) pumped at 1.064 µm and operating in the 1.8–2.5 µm range. This study identifies a set of Sellmeier expansions and thermooptic coefficients that can be used to accurately design...

Nonlinear optical interactions involving counter-propagating photons are of great interest for both classical and quantum optical applications. However, their use is sparsely spread due to the fact that they require quasi-phase-matched devices with sub-µm periods. A recent breakthrough has been the fabrication of bulk sub-µm domain gratings in Rb-d...

A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulati...

The mid-infrared type-II PPRKTP optical parametric oscillators (OPO) operating in 2 μm region were investigated using quasi-phase-matched structures with a range of periodicities. A design precisely targeting on/off greenhouse gas absorption lines was devised.

We present a differential absorption lidar based on a single-frequency parametric source operating at 2 μm which was used for range-resolved measurements of the stable water isotopologues H 2 ¹⁶ O and HD ¹⁶ O.

Precision-tunable mJ, nanosecond pulses at 1981.1 nm and 2145.6 nm are generated at 5kHz repetition with an efficiency of 70% in a cavity-free arrangement using a PPRKTP BWOPO pumped by a narrowband DPSSL at 1030nm.

We demonstrate 3 rd order QPM backward SHG in a bulk PPRKTP with a poling period of 317nm. This is the shortest QPM period ever fabricated. These results pave the way towards 1 st order BSHG.

Precision-tuned nanosecond pulses at 1981.1 nm and 2145.6 nm are generated from a PPKTP BWOPO with 70% efficiency, pumped by an amplified single-frequency fiber laser at 1030 nm. It is free from mechanically-adjustable optical cavities.

We demonstrate 1 st order QPM backward SHG in a bulk PPRKTP with a poling period of 317 nm. Using a narrowband nanosecond-source, the crystal demonstrated a conversion efficiency of 17.6%.

Cavity-free, precision-tunable source of a nanosecond, mJ-level, pulses consisting of backward optical parametric oscillator pumped by diode-seeded laser amplifier shows a record threshold of 19 MW/cm ² and an optical-to-optical efficiency exceeding 70%.

We observed backward-wave spontaneous parametric downconversion in sub- µ m periodically poled Rb-doped KTP. Pumped at 800 nm, forward-wave signal at 1400 nm and backward-wave idler at 1868 nm were obtained.

We investigate frequency doubling of focused Bessel-like higher order fiber modes in a one-dimensionally quasi-phase matched structured KTP crystal. A single higher-order fiber mode, ${{\rm LP}_{0,7}}$ , was generated at 971 nm using a fiber optical parametric amplifier pumped by sub-nanosecond pulses at 1064 nm. Frequency doubling to 485.5 nm was...

We demonstrate multi-cycle terahertz (MC-THz) generation in a 15.5 mm long periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) crystal with a poling period of 300 µm. By cryogenically cooling the crystal to 77 K, up to 0.72 µJ terahertz energy is obtained at a frequency of 0.5 THz with a 3 GHz bandwidth. A maximum internal...

We demonstrate large aperture PPRKTP with a period as short as 3.43 m fabricated via coercive-field engineering through ion-exchange. The key parameters to achieve high-aspect ratio domain structures are discussed.

An ion-exchange process has been developed for periodically poled Rb-doped KTiOPO4 (RKTP) which warrants high efficiency and low loss channel waveguides. The domain stability was investigated, and it was found that domain gratings with uncharged walls could stand the ion-exchange process without deterioration. 3.1 mW of blue second harmonic light w...

We worked on a new scheme of quasi-phase matching (QPM) based on the negative first order of the spatial modulation of the sign of the second-order nonlinearity. Applying this scheme in the case of angular-QPM (AQPM) in a biaxial crystal reveals new directions of propagation for efficient parametric frequency conversion as well as “giant” spectral...

We investigate multi-cycle 0.5-Terahertz generation in a periodically poled Rb-doped potassium titanyl phosphate (PPKTP) crystal. Up to 0.65-gJ terahertz energy was obtained at 77 K with maximum internal optical-to-terahertz efficiency of 0.09%.

We propose a new scheme of quasi-phase matching based on the negative first-order of the spatial modulation of the sign of the second-order optical nonlinearity. The experimental validation is done in a periodically-poled Rubidium-doped KTiOPO4 crystal.

The frequency modulation transfer property of a backward-wave optical parametric oscillator (BWOPO) is investigated in the context of near-IR pulse compression. The maximum transferrable bandwidth from the pump to the forward wave in a BWOPO is determined by the group dispersion mismatch. In comparison, the third-order phase introduced in a single-...

The strict momentum conservation constraints for backward-wave optical parametric oscillators (BWOPOs) gives an inherently narrowband backward-generated wave, even with broadband pumping. Unfortunately, the limited tuning range of this wave restricts potential applications. Here we demonstrate a method to circumvent this restriction and increase th...

The strict momentum conservation constraints for backward-wave optical parametric oscillators (BWOPOs) gives an inherently narrowband backward-generated wave, even with broadband pumping. Unfortunately, the limited tuning range of this wave restricts potential applications. Here we demonstrate a method to circumvent this restriction and increase th...

A way to stabilize the domain structure in periodically poled Rb-doped KTiOPO 4 samples at high-temperatures is presented. The domain contraction along the b-crystallographic axis that is observed when crystals are annealed at high temperatures is suppressed when the ends of the domains along the b-axis are diced away. Additionally, the thermal sta...

The frequency modulation transfer property of a backward wave optical parametric oscillator is employed to generate compressed near-IR pulses at 1.4µm with 1.3ps duration. Limitations to the linear frequency modulation transfer in BWOPO are investigated.

We report the first experimental validation of angular quasi-phase-matching (AQPM) theory in a biaxial crystal by performing second-harmonic generation (SHG) in the periodically-poled Rb-doped (PPRKTP) crystal cut as a sphere. Both AQPM and birefringence phase-matching (BPM) angles were measured thanks to a Kappa circle.

We report on the development of a nanosecond mirrorless optical parametric oscillator (OPO) pumped at 1 μm. The gain medium of the OPO was periodically poled Rubidium-doped KTP with a grating period of Λ = 509 nm for the first order quasi-phase matching. For grating periods of this length, we demonstrate backward propagation of the signal field and...

Ultrafast sources in the mid-IR are indispensable research tools for spectroscopic and medical applications and can also potentially be used to generate attosecond pulses. We investigated a route to directly achieve self-compressed supercontinuum pulses with an octave-exceeding spectrum extending into the mid-IR by employing self-defocusing nonline...

Intracavity difference-frequency generation (DFG) between signal and idler pulses is investigated in orientation-patterned GaAs inside the cavity of a ~1 µm pumped nanosecond optical parametric oscillator (OPO). Using two different samples and temperature tuning in the non-critical configuration, tunability between 7 and 9.2 µm is demonstrated. The...

A noncollinear parametric amplification scheme using the highly nonlinear and damage-resistant GaSe crystal is demonstrated. Amplification bandwidths allowing 10-15fs compressed pulse durations are achieved at 2μm signal wavelength.

We report the first measurements of angular quasi-phase-matching directions of second-harmonic generation performed in the periodically-poled Rb-doped KTiOPO4 biaxial crystal cut as a sphere.

We employ the new chalcogenide crystal BaGa2GeSe6 for the first time for frequency down-conversion into the mid-IR, in an optical parametric oscillator (OPO), intracavity pumped by the signal wave of a 1.064-µm pumped Rb:PPKTP OPO.

Backward-wave OPO with forward mid-infrared idler and backward signal generation is demonstrated in 500-nm-periodicity PPRKTP. The output energy of 2.39mJ and efficiency of 46.4% is obtained. This design has potential for broad tunability in mid-infrared.

We experimentally demonstrate octave-spanning supercontinuum generation and nonlinear pulse compression, down to sub-20-fs, in periodically poled Rb-doped KTP. The results of a detailed numerical study provide excellent agreement with the experiments.

We present the recent development on periodically poled nonlinear crystals with sub-wavelength periodicities as short as 500 nm. These devices show conversion efficiencies larger than 45%. Their unique properties will be presented and discussed.

We performed the first measurements of angular quasi-phase-matching (AQPM) of second-harmonic generation in the periodically-poled Rb-doped KTiOPO4 crystal cut as a sphere, which validates the theory of AQPM in the case of a biaxial crystal.

Mirrorless optical parametric oscillators (MOPOs) are very attractive parametric devices that rely on the nonlinear interaction of counter-propagating photons to inherently establish distributed feedback, without the use of external mirrors or surface coatings. These devices offer unique spectral and coherence properties that will benefit a large v...

We investigate the effect of gamma radiation on flux grown KTiOPO4 and Rb:KTiOPO4 samples, as well as their periodically poled variants. Specifically, we study the altered transmission due to color-center formation via gamma irradiation. We measured the transmission of our samples for varying radiation doses and demonstrate effective temperature an...

A highly efficient mirrorless optical parametric oscillator (MOPO), pumped by narrowband nanosecond pulses at 1064 nm, is demonstrated. The MOPO is based on quasi-phase-matched parametric interaction of counter-propagating photons in 1-mm-thick periodically poled Rb-doped crystal with a period of 755 nm. It generates a co-propagating signal at 1740...

The effect of chemical etching on the stability of domains in periodically poled stoichiometric lithium tantalate is studied by on-line second harmonic generation and microscopy. It is found that wet etching directly after poling leads to domain-wall movement, resulting in back-switching or domain merging. Head-to-head domains tend to backswitch, w...

An overall quantum conversion efficiency of 7.8% is achieved by intracavity mixing the signal and idler of a 1.064 μm pumped Rb:PPKTP optical parametric oscillator in . In this way, a pulse energy of is generated at μm for a repetition rate of 100 Hz. Tuning of the mid-IR radiation is demonstrated by heating of the Rb:PPKTP crystal.

A laser-induced damage study is performed on blank, uncoated Rb:KTiOPO4 (RKTP) samples at 1.03 µm with two different pulse durations of ~0.3 and ~1.0 ps and a repetition rate of 100 kHz. The effect of the sample temperature is also considered.

Strongly enhanced backward stimulated polariton scattering (BSPS) is demonstrated in periodically-poled KTiOPO4 (KTP) crystals with a high power-conversion efficiency up to 70%. We study the physical mechanism of such counter-propagating parametric interaction with phonon-polaritons in χ⁽²⁾ modulated structures. BSPS is a three-wave mixing that is...

We demonstrate a monolithic frequency converter incorporating up to four tuning degrees of freedom, three temperature and one strain, allowing resonance of pump and generated wavelengths simultaneous with optimal phase-matching. With a Rb-doped periodically-poled potassium titanyl phosphate (KTP) implementation, we demonstrate efficient continuous-...

An overall quantum conversion efficiency of 7.8% is achieved by intracavity mixing the signal and idler of a 1.064-µm pumped Rb:PPKTP OPO in BaGa4Se7, generating >0.7 mJ pulse energy at ~7 µm and 100 Hz.